A continuing trend in semiconductor technology is to build integrated circuits with more and/or faster semiconductor devices. The drive toward this ultra large-scale integration (ULSI) has resulted in continued shrinking of device and circuit features. To take advantage of increasing number of devices and to form them into one or more circuits, the various devices need to be interconnected.
To accomplish interconnection on such a small scale, a local interconnect is typically used within an integrated circuit to provide an electrical connection between two or more conducting or semi-conducting regions (e.g., active regions of one or more devices). For example, a plurality of transistors can be connected to form an inverting logical circuit using a local interconnect.
The local interconnect is typically a relatively low-resistance material, such as a conductor or doped semiconductor, that is formed to electrically couple the selected regions. For example, in certain arrangements, damascene techniques are used to provide local interconnects made of tungsten (W), or a like conductor, which is deposited within an etched opening, such as via or trench that connects the selected regions together. The use of local interconnects reduces the coupling burden on the subsequently formed higher layers to provide such connectivity, which reduces the overall circuit size and as such tends to increase the circuit's performance.
The formation of the etched openings, in which the local interconnects are formed, requires precise process controls in order to avoid over-etching and/or under-etching within the various layers of the semiconductor wafer. There are at least two conventional methods for determining when to terminate an etching process. One method requires collecting experimental data from test wafers to determine an optimal endpoint. Once the optimal endpoint is determined, then the etching process for subsequent wafers is simply timed to terminate at the specified endpoint. For example, the etching process may be conducted for 20 seconds. However, this timed method is subject to failure due to minor differences in the wafers, the etching tools and/or the etching process parameters.
Another conventional method for determining the endpoint involves monitoring an optical emission from the etching process. This method allows for real-time feedback during the etching process based on differences in the wafer's materials and the reaction between these materials and the etching plasma. By monitoring a specific wavelength from the optical emissions, it is possible to detect transition points wherein the etching plasma begins to react with different materials. This method usually provides better process control when compared to the timed method. However, in order to take advantage of this optical emission monitoring method, a particular wavelength needs to be identified for the particular wafer structure and/or the etching plasma. Thus, for example, an ideal wavelength would identify when a transition from one material/reaction to another has occurred, or is occurring. Finding such wavelengths, if they exist, is not a trivial task. Assuming a wavelength is found to be emitted during the etching process, it can also be difficult to sense the wavelength and to isolate the wavelength from the optical noise within the etching tool. Thus, false readings of this wavelength can lead to incorrect endpoint determination and therefore under-etching or over-etching of the wafer.
Accordingly, there is a continuing need for more efficient and effective processes for forming local interconnects using damascene techniques. In particular, there is a need for more precise etching process controls for determining the endpoint in forming etched openings using optical emission monitoring methods.